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<h3><a name="SECTION0022167000000000000000">
20.1.6.7 Bindings and Events</a>
</h3> 
<p>
<a id='l2h-4706' xml:id='l2h-4706'></a>

<p>
The bind method from the widget command allows you to watch for
certain events and to have a callback function trigger when that event
type occurs.  The form of the bind method is:

<p>
<div class="verbatim"><pre>
    def bind(self, sequence, func, add=''):
</pre></div>
where:

<p>
<dl>
<dt><strong>sequence</strong></dt>
<dd>is a string that denotes the target kind of event.  (See the bind
man page and page 201 of John Ousterhout's book for details).

<p>
</dd>
<dt><strong>func</strong></dt>
<dd>is a Python function, taking one argument, to be invoked when the
event occurs.  An Event instance will be passed as the argument.
(Functions deployed this way are commonly known as <var>callbacks</var>.)

<p>
</dd>
<dt><strong>add</strong></dt>
<dd>is optional, either "<tt class="samp"></tt>" or "<tt class="samp">+</tt>".  Passing an empty string
denotes that this binding is to replace any other bindings that this
event is associated with.  Preceeding with a "<tt class="samp">+</tt>" means that this
function is to be added to the list of functions bound to this event type.
</dd>
</dl>

<p>
For example:
<div class="verbatim"><pre>
    def turnRed(self, event):
        event.widget["activeforeground"] = "red"

    self.button.bind("&lt;Enter&gt;", self.turnRed)
</pre></div>

<p>
Notice how the widget field of the event is being accessed in the
<tt class="method">turnRed()</tt> callback.  This field contains the widget that
caught the X event.  The following table lists the other event fields
you can access, and how they are denoted in Tk, which can be useful
when referring to the Tk man pages.

<p>
<div class="verbatim"><pre>
Tk      Tkinter Event Field             Tk      Tkinter Event Field 
--      -------------------             --      -------------------
%f      focus                           %A      char
%h      height                          %E      send_event
%k      keycode                         %K      keysym
%s      state                           %N      keysym_num
%t      time                            %T      type
%w      width                           %W      widget
%x      x                               %X      x_root
%y      y                               %Y      y_root
</pre></div>

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